The arrangement of transducers in an array for an interferometric system is usually based on physical conveniences or other external considerations. Such systems may be used to determine direction of arrival of radiation received from a remote source. When used for this purpose, the arrangement can result in phase ambiguities which affect resolution.
The prior art contains several methods of phase ambiguity resolution for interferometric systems consisting of either collinear or non-collinear, coplanar arrangements of transducers, such as antennas.
In a treatise published in 1973 by James E. Hanson titled “On Resolving Angle Ambiguities of n-Channel Interferometer Systems for Arbitrary Antenna Arrangements In a Plane” (Defense Technical Information Center Publication Number AD 776-335) addresses ambiguities. In this treatise, Hanson demonstrated how the problem of interferometric phase ambiguity resolution could be easily approached by casting the several differential phase measurements into direction cosine space as a set of equally spaced parallel straight lines.
According to Hanson, phase ambiguity resolution is accomplished by finding an arrangement of three or more antennas that create a Hanson ambiguity diagram with but a single point of intersection of the various trajectories, an intersection that is located in direction cosine space at the exact position of the radiating source; for strictly collinear arrays of antennas the single intersection is rather a single straight line. It is also noted that this single point of intersection in direction cosine space leads immediately to the two angles of arrival—φthe azimuth angle and θ the zenith angle—so that ambiguity resolution leads to the determination of the angles of arrival.
The differential phase measurements made with practical interferometers come with errors that arise due to systematic as well as thermodynamic perturbations within the array antennas and the receiving network. These errors cause the Hanson trajectories to move or shift randomly at right angles to the directions in which they lay. As a consequence, in order to determine the single point of intersection in the ideal, no error condition becomes a set of pair-wise trajectory intersections. Thus, ambiguity resolution is accomplished by designing the ambiguity resolution computer algorithm so that it can discern a tightly grouped set of pair-wise intersections. Such an approach is described by Azzarelli, et al. in U.S. Pat. No. 6,140,963 but only for non-linear, coplanar arrays.
However, there is a need for a system and method which deal with the arrangement of transducers and particularly non-coplanar arrangements of antenna elements in order to minimize phase ambiguities and in order to maximize the determination of direction of arrival or angle of arrival of a signal received by the arrangements and emitted by a source remote from the arrangements. In addition, there is a need for a method which minimizes the phase errors that arise due various perturbations.